1. Field of the Invention
The invention relates to a catadioptric projection objective for imaging an off-axis effective object field arranged in an object surface of the projection objective onto an off-axis effective image field arranged in an image surface of the projection objective.
2. Description of the Related Art
Catadioptric projection objectives are, for example, employed in projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of micro-devices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order to create even finer structures, it is sought to both increase the image-end numerical aperture (NA) of the projection objective and employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm. However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, that are sufficiently transparent in that wavelength region available for fabricating the optical elements. Since the Abbe numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations).
The high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems. Measures that allow reducing the number and sizes of lenses employed and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
In optical lithography, high resolution and good correction status have to be obtained for a relatively large, virtually planar image field. It has been pointed out that the most difficult requirement that one can ask of any optical design is that it have a flat image, especially if it is an all-refractive design. Providing a flat image requires opposing lens powers and that leads to stronger lenses, more system length, larger system glass mass, and larger higher-order image aberrations that result from the stronger lens curvatures. Conventional means for flattening the image field, i.e. for correctings the Petzval sum in projection objectives for microlithography are discussed in the article “New lenses for microlithography” by E. Glatzel, SPIE Vol. 237 (1980), pp. 310-320.
Concave mirrors have been used for some time to help solve problems of color correction and image flattening. A concave mirror has positive power, like a positive lens, but the opposite sign of Petzval curvature. Also, concave mirrors do not introduce color problems. Therefore, catadioptric systems that combine refracting and reflecting elements, particularly lenses and one or more concave mirrors, are primarily employed for configuring high-resolution projection objectives of the aforementioned type. Unfortunately, a concave mirror is difficult to integrate into an optical design, since it sends the radiation right back in the direction it came from. Intelligent designs integrating concave mirrors without causing mechanical problems or problems due to beam vignetting or pupil obscuration are desirable.
A further design goal is to optimize the size and shape of the object field which can be effectively imaged by the projection objective without vignetting at a given numerical aperture. The corresponding object field will be denoted in “effective object field” in the following. The size of the effective object field and the size of the corresponding effective image field are related through the magnification factor of the projection objective. Often it is desired to maximize the size of the effective fields in order to improve productivity of manufacturing processes involving the projection objective. For a given object-side numerical aperture, the size of an effective object field corresponds to an “effective geometrical light conductance value” (or “effective etendue”), which is defined herein as the product of the object-side numerical aperture and the radius REOF of a circle having minimum size including the effective object field.
A further parameter to be observed when designing a projection objective is the size of the object field for which the projection objective must be sufficiently corrected with respect to image aberrations in order to obtain the desired performance. The aberrations include chromatic aberrations, image curvature aberration, distortion, spherical aberrations, astigmatism etc. The field, for which the projection objective must be sufficiently corrected, will be denoted “design object field” in the following. As the number and sizes of optical elements typically increase drastically if the size of the design object field is to be increased, it is generally desired to minimize the size of the design object field. At a given object-side numerical aperture, a projection objective may be characterized by a specific value for the “design etendue”, which is defined in this application as the product between the object-side numerical aperture and the outer radius RDOF of the design object field, i.e. the design object field radius.
In purely refractive projection objectives an effective object field centered around the optical axis can be used. Likewise, a centered effective object field can be used in catadioptric projection objectives having a physical beam splitter, e.g. a beam splitter having a polarization selective beam splitter surface, or in systems having central pupil obscuration. In such systems the effective etendue equals the design etendue (i.e. REOF=RDOF) indicating that an optimum size effective field can be used with a minimum size of the design object field. However, as colour correction becomes increasingly difficult in high aperture refractive projection objectives and since designs having physical beam splitters may be difficult to handle in terms of polarization control, alternative catadioptric designs (off-axis systems) have been developed, which can be subdivided into designs using geometrical beam splitting with one or more planar folding mirrors and so-called “in-line systems” having one straight (unfolded) optical axis common to all optical elements.
In these off-axis systems, an off-axis effective object field, i.e. an effective object field positioned entirely outside the optical axis, must be used to avoid vignetting. Both rectangular effective object fields and effective object fields having an arcuate shape, typically denoted as “annular field” or “ring field” have been proposed in this type of designs.
Representative examples for folded catadioptric projection objectives using planar folding mirrors in combination with a single catadioptric group having a concave mirror near or at a pupil surface and negative refractive power in front of the concave mirror are given in US 2003/0234912 A1 or US 2004/0160677 A1. These types of designs are typically used with a rectangular effective object field.
Various catadioptric in-line projection objectives have been proposed, From an optical point of view, in-line systems may be favorable since optical problems caused by utilizing planar folding mirrors, such as polarization effects, can be avoided. Also, from a manufacturing point of view, in-line systems may be designed such that conventional mounting techniques for optical elements can be utilized, thereby improving mechanical stability of the projection objectives.
The patent U.S. Pat. No. 6,600,608 B1 discloses a catadioptric in-line projection objective having a first, purely refractive objective part for imaging a pattern arranged in the object plane of the projection objective into a first intermediate image, a second objective part for imaging the first intermediate image into a second intermediate image and a third objective part for imaging the second intermediate image directly, that is without a further intermediate image, onto the image plane. The second objective part is a catadioptric objective part having a first concave mirror with a central bore and a second concave mirror with a central bore, the concave mirrors having the mirror faces facing each other and defining an intermirror space or catadioptric cavity in between. The first intermediate image is formed within the central bore of the concave mirror next to the object plane, whereas the second intermediate image is formed within the central bore of the concave mirror next to the object plane. The objective has axial symmetry and a field centered around the optical axis and provides good color correction axially and laterally. However, since the reflecting areas of the concave mirrors exposed to the radiation are interrupted at the bores, the pupil of the system is obscured.
The Patent EP 1 069 448 B1 discloses catadioptric projection objectives having two concave mirrors facing each other and an off-axis object and image field. The concave mirrors are part of a first catadioptric objective part imaging the object onto an intermediate image positioned adjacent to a concave mirror. This is the only intermediate image, which is imaged to the image plane by a second, purely refractive objective part. The object as well as the image of the catadioptric imaging system are positioned outside the intermirror space defined by the mirrors facing each other. Similar systems having two concave mirrors, a common straight optical axis and one intermediate image formed by a catadioptric imaging system and positioned besides one of the concave mirrors is disclosed in US patent application US 2002/0024741 A1.
US patent application US 2004/0130806 (corresponding to European patent application EP 1 336 887) discloses catadioptric projection objectives having off-axis object and image field, one common straight optical axis and, in that sequence, a first catadioptric objective part for creating a first intermediate image, a second catadioptric objective part for creating a second intermediate image from the first intermediate image, and a refractive third objective part forming the image from the second intermediate image. Each catadioptric system has two concave mirrors facing each other. The intermediate images lie outside the intermirror spaces defined by the concave mirrors.
Japanese patent application JP 2003114387 A and international patent application WO 01/55767 A disclose catadioptric projection objectives with off-axis object and image field having one common straight optical axis, a first catadioptric objective part for forming an intermediate image and a second catadioptric objective part for imaging the intermediate image onto the image plane of this system. Concave and convex mirrors are used in combination.
US patent application US 2003/0234992 A1 discloses catadioptric projection objectives with off-axis object and image field having one common straight optical axis, a first catadioptric objective part for forming an intermediate image, and a second catadioptric objective part for imaging the intermediate image onto the image plane. In each catadioptric objective part concave and convex mirrors are used in combination with one single lens.
International patent application WO 2004/107011 A1 discloses various catadioptric projection objectives with off-axis object field and image field having one common straight optical axis designed for immersion lithography using an arc shaped effective object field having a field center far away from the optical axis. The projection objectives include various types of mirror groups having two, four or six curved mirrors. Embodiments with two to four intermediate images are disclosed. US patent application US 2004/0218164 A1 discloses illumination systems designed for providing an arc shaped illumination field. A combination with a catadioptric projection objective with a polarization-selective physical beam splitter is also disclosed.
US provisional application with Ser. No. 60/536,248 filed on Jan. 14, 2004 by the applicant discloses a catadioptric projection objective having very high NA and suitable for immersion lithography at NA>1. The projection objective comprises: a first objective part for imaging the pattern provided in the object plane into a first intermediate image, a second objective part for imaging the first intermediate imaging into a second intermediate image, and a third objective part for imaging the second intermediate imaging directly onto the image plane. The second objective part includes a first concave mirror having a first continuous mirror surface and a second concave mirror having a second continuous mirror surface, the concave mirror surfaces facing each other and defining an intermirror space. All mirrors are positioned optically remote from a pupil surface. The system has potential for very high numerical apertures at moderate lens mass consumption.